Glass masks for photoetching used in the production of integrated circuits are required to have a high optical homogeneity free of defects such as bubbles, cords and contaminants. They are also required to have a high stability and, for obtaining a high degree of integration in the integrated circuits, a small coefficient of thermal expansion. Further, they must have good heat resisting and refractory properties so as not to cause breakage or deformation in the heating cycle in the process for vapor-depositing metal on the mask, excellent chemical durability to washing and etching by various chemicals, an excellent ultraviolet transmittance and good adhesion of a metal vapor-deposited film with which an integrated circuit pattern is formed on the mask.
Known in the art of glasses for photomask is a Pyrex type boro-silicate glass. The glass, however, requires a high temperature in melting and is difficult to homogenize. Besides, this glass is inferior in the adhesion of the metal vapor-deposited film due to its high alkali content and is not sufficient in the refractory property, softening temperature being less than about 650.degree. C. Known also in the art are calcium-alumino-silicate glasses of the fibre E and refractory glass types. These glasses have improved melt properties over the above described boro-silicate glass but have the disadvantage that they must contain a relatively large amount of CaO with a result that the heat resisting properties are unsatisfactory with a relatively large coefficient of thermal expansion of about 42 to 65 (.alpha..times.10.sup.7).
There are also disclosed SiO.sub.2 -Al.sub.2 O.sub.3 -CaO-MgO-ZnO-PbO glass compositions in the specifications of U.S. Pat. Nos. 4,391,916 and 4,403,043. These glasses also must contain a large amount of CaO for maintaining the stability of the glass resulting in a relatively large coefficient of thermal expansion which is more or less similar to the calcium-alumino silicate glass.